Gypsum, which is calcium sulfate, has wide application in the manufacture of construction products and particularly wallboard. Gypsum is obtained from two different sources, the predominant source being natural gypsum. Natural gypsum is mined and the particulate or rock form gypsum is ground to a powdery state wherein it is heated and mixed with other additives, etc. Natural gypsum is preferred by manufacturers of wallboard and other commercial products to synthetic gypsum because it lends itself to ease of handling and to the preparation of fine quality wallboard.
Synthetic gypsum is generally obtained as a byproduct in the manufacture of phosphate containing fertilizers and as a byproduct in the desulfurization of flue gas. Neither byproduct has experienced widespread commercial succeeds for the formation of quality products. Usually synthetic gypsum has represented a disposal problem rather than a commercial product to those associated with the byproduct manufacture of calcium sulfate. Synthetic gypsum has a different crystal size and shape than natural gypsum and that difference has been used as a basis for explaining the difference in physical properties obtained in commercial products.
Desulfurization of flue gas is accomplished by one process known as the "wet process with forced oxidation" wherein the flue gas containing sulfur dioxide is contacted with calcium carbonate (limestone) or lime generating an intermediate calcium sulfite product. The interim calcium sulfite is oxidized to calcium sulfate dihydrate by contacting the mixture of calcium carbonate and calcium sulfite with an oxygen containing gas under oxidizing conditions. Gypsum, calcium sulfate dihydrate, obtained from flue gas desulfurization by the wet limestone process is in the beta crystal form which is sometimes a crystal form of hemihydrate or anhydrite. The powder has extremely poor flow characteristics, it agglomerates or sticks together and does not exhibit free flow in storage hoppers and bins. As a result of these poor handling properties, synthetic calcium sulfate dihydrate obtained as a byproduct from the desulfurization of flue gas by the wet limestone process is not well suited for the manufacture of wallboard or well suited for other applications which require an easily processible material.
There is substantial art regarding the generation of gypsum calcium sulfate dihydrate obtained as a byproduct from industrial processes. Representative patents which address the recovery of gypsum calcium sulfate from industrial processes include the following:
U.S. Pat. No. 3,820,970 discloses a process for producing gypsum granules or pellets resistant to dusting by admixing calcium sulfate dihydrate with from about 10-20% of calcium sulfate hemihydrate and 2-4% water. The substantially dry solid-liquid mixture is compacted at pressures of 1,000-2,000 psi, and the resulting sheet material broken to form flakes and the flakes crushed to the desired granular size. Higher levels of water were alleged as resulting in very dusty and friable granules.
U.S. Pat. No. 4,173,610 discloses a process for producing pelletized calcium sulfate from finely divided natural or synthetic calcium sulfate. The patentees point out that it was typical in prior art processes to transform finely divided calcium sulfate into lump calcium sulfate by addition of binding agents such carboxymethyl cellulose or crystal modifying substance. In producing lump calcium sulfate in a single operation, the finely divided base material is adjusted to have a free water content of from about 0.5 to 4% by weight and then compacted in a roll press to form pellets or a lump type material. Temperatures of compaction range from about 0.degree. to 60.degree. C. and roll pressures range from about 1 to 5 metric tons per centimeter (Mp) of roller length.
U.S. Pat. No. 4,544,542 discloses a process for producing pelletized gypsum from a dry flue gas desulfurization process. Small amounts of low-melting constituents such as sodium carbonate, sodium silicate or calcium chloride are added to the precursor calcium sulfite and then oxidizing the calcium sulfite material to calcium sulfate. The low melting constituents in the liquid phase then agglomerate with the calcium sulfate to form pellets.
U.S. Pat. No. 4,377,414 discloses a process for the production of pellets of gypsum using the desulfurization product obtained from flue gas desulfurization using the dry scrubbing technique in combination with fly ash. In forming the pellets, a fly ash containing powder is admixed with microparticulate crystallites of desulfurized scrubber material, e.g., calcium sulfate and then mixed with water to from a powder/water mixture. This mixture is shaped under compaction and cured. The product can be characterized as fly ash particles coated with desulfurization product and unreacted agent.
U.S. Pat. No. 4,954,134 discloses a process for producing pellets of gypsum obtained from a feedstock of phosphogypsum. The process utilizes a granulating/dispersion aide comprising a lignosulfonate solution.
Summarizing the prior art, calcium sulfate obtained by the desulfurization of flue gas utilizing the wet limestone process has had limited commercial acceptance and encountered many difficulties in conversion to materials suitable for construction. There is substantially no market for this type of gypsum calcium sulfate. This product in finely divided powdery form, has poor flow, it is sticky, cohesive and it has poor handling characteristics. It also has limited structural load-bearing capabilities.